This project will be a continuation and an extension of current studies on the genetic control of the immune response to the Thy-1.1 antigen. The magnitude of the immune response of mice to this antigen will be measured by the number of cells producing antibodies lytic for cells carrying the antigen under investigation. Cells producing antibodies will be detected and enumerated in the spleens of immunized animals employing the technique of cytolysis in agar gel. Cellular requirements for the primary immune response to the Thy-1.1 antigen will be studied using a technique of cell transfer into lethally irradiated recipients. Subsequently, attempts will be made to define the cellular basis of the genetic low responsiveness utilizing as the donors of grafted cells animals differing in their responsiveness. Employing Ir-5 congenic line, presently raised in this laboratory, attempts will be made to produce antibodies directed to the products of the Ir-5 locus. These antibodies will be used to determine the cellular distribution of the Ir-5 products as well as the strain distribution. The number and localization of the H-2 associated loci determining the responsiveness to the Thy-1.1 antigen will be studied utilizing the H-2 recombinants. In addition, studies of the genetic control of the immune response to the Thy-1.2 antigen will be continued.